Date:	Mon, 6 Dec 2010 09:09:55 +0100
From:	"Robert Lukassen" <Robert.Lukassen@...tom.com>
To:	"Jef Driesen" <jefdriesen@...enet.be>, "Greg KH" <greg@...ah.com>
Cc:	"David Brownell" <dbrownell@...rs.sourceforge.net>,
	"Greg Kroah-Hartman" <gregkh@...e.de>, <linux-usb@...r.kernel.org>,
	<linux-kernel@...r.kernel.org>
Subject: RE: [PATCH] USB: g_serial: Allow to override the default VID/PID

Hi,

Yes, providing access to those module parameters was the original goal. I agree that my patch should be reverted and I'll try to come up with a method of accessing these values without violating the information hiding principles too much. I don't like accessing globals.

Robert

> -----Original Message-----
> From: Jef Driesen [mailto:jefdriesen@...enet.be] 
> Sent: Friday, December 03, 2010 11:10 PM
> To: Greg KH
> Cc: Robert Lukassen; David Brownell; Greg Kroah-Hartman; 
> linux-usb@...r.kernel.org; linux-kernel@...r.kernel.org
> Subject: Re: [PATCH] USB: g_serial: Allow to override the 
> default VID/PID
> 
> On 30/11/10 18:46, Greg KH wrote:
> > On Mon, Nov 22, 2010 at 11:20:55AM +0100, Robert Lukassen wrote:
> >> Hi,
> >>
> >> I just revisited the patch I submitted and I can see that it needs 
> >> fixing. The intention of the patch was to provide the idVendor& 
> >> idProduct kernel module parameter overrides to the individual 
> >> functions at 'bind()' time. Prior to the patch, the overrides were 
> >> done after the bind, making it hard for individual functions to 
> >> determine the correct value for idVendor (some functions 
> may need to 
> >> use that value, e.g. USB audio when it registers itself as an ALSA 
> >> device; it would want to use the same Vendor id).
> >>
> >> The patch makes sure that that happens, however, right after the 
> >> bind() the patched values are lost due to the copying of 
> the gadget 
> >> specific device descriptor. Here's the snippet from 2.6.34:
> >>
> >>   999        usb_gadget_set_selfpowered(gadget);
> >> 1000
> >>
> >> 1001        /* interface and string IDs start at zero via kzalloc.
> >> 1002         * we force endpoints to start unassigned; few 
> controller
> >> 1003         * drivers will zero ep->driver_data.
> >> 1004         */
> >> 1005        usb_ep_autoconfig_reset(cdev->gadget);
> >> 1006
> >> 1007        /* composite gadget needs to assign strings 
> for whole device (like
> >> 1008         * serial number), register function drivers, 
> potentially update
> >> 1009         * power state and consumption, etc
> >> 1010         */
> >> 1011        status = composite->bind(cdev);
> >> 1012        if (status<  0)
> >> 1013                goto fail;
> >> 1014
> >> 1015        cdev->desc = *composite->dev;
> >> 1016        cdev->desc.bMaxPacketSize0 = gadget->ep0->maxpacket;
> >> 1017
> >> 1018        /* standardized runtime overrides for device ID data */
> >> 1019        if (idVendor)
> >> 1020                cdev->desc.idVendor = cpu_to_le16(idVendor);
> >> 1021        if (idProduct)
> >> 1022                cdev->desc.idProduct = cpu_to_le16(idProduct);
> >> 1023        if (bcdDevice)
> >> 1024                cdev->desc.bcdDevice = cpu_to_le16(bcdDevice);
> >> 1025
> >>
> >> In line 1015, the composite device descriptor is copied to the 
> >> cdev->desc field. Then the values get patched.
> >>
> >> My patch just moved lines 1018-1024, so that the functions could 
> >> access the patched values. The ill side effect is that 
> these patched 
> >> values are lost when the *composite->dev (== device descriptor) is 
> >> copied in line 1015.
> >>
> >> It would seem that the patch should also have moved lines 
> 1015-1016 
> >> in front of the composite->bind(cdev) call.
> >>
> >> It would require all composite gadgets (such as serial.c) and all 
> >> functions (such as f_acm.c) to do any modifications to the device 
> >> descriptor as passed to them in the 'bind' call. For 
> functions that 
> >> is natural, as they should be unaware of the gadget that uses them.
> >>
> >> The composite gadgets such as 'serial' seem to deal with 
> it differently.
> >> The serial.c gadget refers to the device descriptor as 
> 'device_desc', 
> >> and it is a static struct. It references it in its 'init' function 
> >> (which is ok, it is prior to the registration point) and in the 
> >> gs_bind() call. At that point it should probably not be 
> using 'device_desc', but cdev->desc.
> >>
> >> I think we can go one of two ways. I'd be happy to go over the 
> >> composite gadgets and change their use of their device 
> descriptors, 
> >> such that they use cdev->desc at
> >> bind() time. Combining that with the moving the copying of the 
> >> *composite->dev to before calling 'composite->bind()' 
> would make things consistent.
> >>
> >> The other way is reverting my original patch. For those functions 
> >> that need access to the overridden idVendor/idProduct code, we'll 
> >> need to work out another
> >> (generic) solution.
> >
> > So, do you want me to revert your original patch and wait 
> for you to 
> > come up with a "better" solution?
> >
> > That seems like the correct thing to do at the moment.
> 
> If the original commit is reverted, the idVendor and 
> idProduct module parameters can still be accessed through the 
> global variables directly (like I did in my own pathc). Since 
> access to those values appears to be the main goal of the 
> patch, I think no functionality will be lost by reverting the commit.
> 


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